This natural process is used technically in biogas plants to generate biogas from biologically degradable feedstocks, such as liquid manure, bio-waste or energy crops (maize, grass, sugar beet etc.). These feedstocks are fermented in hermetically sealed fermentation tanks, so-called digesters, where they are converted into biogas in a four-stage process. This requires the work of many different micro-organisms.
The most important component of biogas is combustible methane (CH4), which is also a major component of natural gas. Depending on the feedstocks used, the methane content in biogas varies between 50 and 65%. The second major component of biogas is carbon dioxide (CO2) with a content of 35-50%. Apart from that, other substances, such as nitrogen, water, oxygen and hydrogen sulphide, may also be found in the biogas in low concentrations.
The fermentation product retains the nutrients contained in the source feedstocks, such as nitrogen, phosphorus, potassium and the organic carbon. The utilization of fermentation products thus closes the natural nutrient and humus cycle and replaces mineral fertilizer, which must otherwise be produced with a high input of energy or be extracted.